In Japan, only about 30% of the primary energy supply is used as effective energy, with about 70% being eventually lost to the atmosphere as heat. The heat generated by combustion in industrial plants, garbage-incineration facilities and the like is lost to the atmosphere without being converted into other energy. In this way, we are wastefully discarding a vast amount of thermal energy, while acquiring only a small amount of energy by combustion of fossil fuels or other means.
To increase the proportion of energy to be utilized, the thermal energy currently lost to the atmosphere should be effectively used. For this purpose, thermoelectric conversion, which directly converts thermal energy to electrical energy, is an effective means. Thermoelectric conversion, which utilizes the Seebeck effect, is an energy conversion method for generating electricity by creating a difference in temperature between both ends of a thermoelectric material to produce a difference in electric potential.
In such a method for generating electricity utilizing thermoelectric conversion, i.e., thermoelectric generation, electricity is generated simply by setting one end of a thermoelectric material at a location heated to a high temperature by waste heat, and the other end in the atmosphere, and connecting external resistances to both ends. This method entirely eliminates the need for moving parts, such as the motors or turbines generally required for power generation. As a consequence, the method is economical and can be carried out without releasing the gases by combustion. Moreover, the method can continuously generate electricity until the thermoelectric material has deteriorated. Furthermore, thermoelectric generation enables power generation at a high power density. Therefore, it is possible to make electric power generators (modules) small and light enough to be used as mobile power supplies for cellular phones, notebook computers, etc.
Therefore, thermoelectric generation is expected to play a role in the resolution of future energy problems. To realize thermoelectric generation, a thermoelectric module comprising a thermoelectric material that has both a high thermoelectric conversion efficiency and excellent properties in terms of heat resistance, chemical durability, etc., will be required.
CoO2-based layered oxides such as Ca3Co4O9 have been reported as substances that achieve excellent thermoelectric performance in air at high temperatures, and such thermoelectric materials are currently being developed (see Non-Patent Document 1, for example).
However, the development of a thermoelectric module (electric power generator) that is needed to realize efficient thermoelectric generation using thermoelectric materials has been delayed so far.
Non-Patent Document 1: R. Funahashi et al., Jpn. J. Appl. Phys., 39, L1127 (2000)